Phase change analysis in logging method

ABSTRACT

An improved method of fluid analysis in the borehole of a well. A fluid sampling tool is fitted with a pumpout module that can be used to draw fluids from the formation, circulate them through the instrument, and then expel this fluid to the borehole. It has been determined that certain measurements would be most valuable to implement down hole, such as the formation fluid bubble point and dew point. Accurate bubble point and dew point measurements are made by forming bubbles or a liquid drop in a measured sample, and detecting same.

FIELD OF THE INVENTION

The present invention relates to down hole fluid sampling tools andmethods and, more particularly, to an improved fluid extraction tool andmethod for analyzing thermodynamic phases of complex fluids down hole.

BACKGROUND OF THE INVENTION

Schlumberger Technology Corporation, the assignee of the presentinvention, has pioneered the use of Modular Formation Dynamics Testers(MDTs) and other down hole tools. The Modular Formation Dynamics Testeris one of several very useful instruments for obtaining formation fluidsamples. The MDT tool is suspended by a wire line and then lowered intothe borehole of the well. The instrument is secured to the walls of theborehole and samples of the formation fluid are extracted. Such a toolis illustrated in U.S. Pat. No. 4,860,581, issued to Zimmerman et al onAug. 29, 1989.

Fluid sampling tools comprise a pumpout module that can be used to drawfluids from the formation, circulate them through the instrument foranalysis, and then expel these fluids to the borehole. The MDT can alsoretain samples of formation fluids in sampling bottles, which are thentransported to the surface. The samples are transferred at the surfacefrom the sampling bottles to transportation bottles. The formation fluidsamples are then sent to pressure-volume-temperature laboratories (PVTlabs) for analysis of their composition and their physical properties.Conventional PVT labs provide a broad range of measurements andservices.

It is essential to know the bubble point of crude oil, because when theborehole pressure drops below the bubble point pressure duringproduction, gas bubbles form in the porous rock reservoir. Thisdramatically decreases the oil phase relative permeability. Knowledge ofthe bubble point is also useful in determining the composition of thehydrocarbon mixture in the reservoir.

The best current practice of measuring bubble point is to bring a sampleof fluid to the surface to be sent to a laboratory. There, the sample isplaced in a cylinder, the volume of which is increased by a piston.Pressure is monitored by a gauge. The bubble point is normallyconsidered to be the pressure at which a break (knee) appears in thepressure versus volume (P-V) curve.

However, this technique has several disadvantages. It is time consumingto bring a fluid sample to the surface, transfer it to the (possiblydistant) laboratory, and await the result. Further limitations of thistechnique are: (1) only a few samples (typically six or fewer) can betransported to the surface on each tool run; (2) samples are altered bypressure and/or temperature changes when they are brought to thesurface; (3) sample composition can change as a result of imperfecttransfer from sampling bottle to transportation bottle, and tolaboratory apparatus; (4) typically, a delay of several weeks occursbetween the time of fluid sampling and the receipt of the laboratoryreport; (5) it is not known whether the sample and data are valid untillong after the opportunity to take further samples passes; (6) highpressure, toxic, explosive samples must be transported, handled bywellsite and laboratory personnel, and disposed of, creating numerouspotential health, safety and environmental problems.

The break in the aforementioned P-V curve is unreliable for determiningthe bubble point. A more reliable method is to observe bubble formationin the cylinder by use of a sight glass. In this manner, bubbles may bedetected visually. They may also be measured by the transmission of nearinfrared light, since the bubble point is associated with attenuation ofthe light beam.

A number of down hole measurement techniques have been proposed formaking a bubble point measurement within a down hole tool. These methodsare described in U.S. Pat. Nos. 5,329,811; 5,473,939; 5,587,525;5,622,223; and 5,635,631.

As described in the above-mentioned patents, fluid is isolated in theflow line, and then a pump (the same one used to extract fluid from theformation) is used to expand the volume. A pressure gauge is used tomonitor the P-V curve.

Several problems exist with these prior art methods of determiningbubble point. First, the measurement is very time consuming. At eachstage of the expansion, it is necessary to allow bubbles to nucleate.

In U.S. Pat. No. 5,635,631, a gas is formed slowly, “relative to theamount of time taken to expand the sample.” A full bubble pointdetermination can require over an hour. Identifying a single pressure,following the maximum expansion, as the bubble point pressure, isclearly inaccurate since it assumes that the compressibility of thehydrocarbon below the bubble point pressure is negligible. Thisassumption is erroneous, and can lead to substantial errors in bubblepoint pressure determination.

To detect phase changes of complex hydrocarbon mixtures, it is necessaryto nucleate bubbles or drops of the new phase and to detect thesebubbles. In standard laboratory apparatus, and in prior art down holetools, the bubbles or drops are formed at arbitrary locations in thefluid volume, and then detected by pressure-volume measurements, or bydetecting bubbles at another site (e.g., in the beam between a sourceand detector of light). Both of these methods are characterized by adelay between the arrival at the thermodynamic phase line and theinitiation of phase change, and then a delay between the phase changeand its detection. The methods and tools of this invention solve bothproblems.

In a related prior art publication [SPE 30610 (1995) Michaels (Western)]a technique is described in which the volume is increased as thepressure is monitored. Special significance is attached to the pressureat which the P-V curve departs from linearity. The authors cautiouslydeclined to call this pressure the bubble point. This criterion may aidin collecting a sample for surface analysis, but it is not helpful inplanning reservoir operations. This pressure may underestimate thebubble point, if the appearance of bubbles is delayed by retardednucleation. Thus, maintaining the production pressure at this levelduring oil production may lead to formation of gas in the formation, andthus reduced productivity.

The present invention addresses a method of providing a down hole methodof making rapid, accurate measurements of bubble point using a down holetool, such as an MDT tool.

The dew point is the most important thermodynamic parameter associatedwith gas condensate reservoirs. Gas condensate reservoirs produce gas athigh pressure. As the pressure drops, liquid is formed. When thishappens in the pore space of the rock, the permeability to gas flow isgreatly reduced, with accompanying loss of production. Therefore, it isimportant to maintain the pressure of gas condensate reservoirs abovethe dew point for as long as possible.

Sensors have been developed to measure the dew point of ordinary humidair. A cooled plate provides a definite location for the nucleation ofliquid drops. The plate is part of a mass-sensitive sensor, such as anacoustic surface wave resonator, which detects the first presence of theliquid. H. Ziegler and K. Rolf, “Quartz Sensor for Automatic Dew-PointHygrometry”, Sensors and Actuators, Vol. 11, pp. 37-44 (1987).

Devices of this kind will often fail when used to measure the dew pointof gas condensates under down hole conditions. This is so, becausemixtures of hydrocarbons found in reservoirs can have unusual phasediagrams. As the pressure is reduced, the first condensation of liquidcan occur at either the hottest or the coldest point accessible to themixture. Amyx, Bass, Whiting, “Petroleum Reservoir Engineering”,McGraw-Hill, 1960, pp. 220-229.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided an improvedmethod of fluid analysis in the borehole of a well. Fluid sampling toolsas well as other down hole tools can be used to measure the bubble pointand dew point of the extracted fluids. For example, an MDT toolcomprises a pumpout module that can be used to draw fluids from theformation, circulate them through the instrument, and then expel thisfluid to the borehole. It has been determined that bubble point and dewpoint measurements can be measured accurately with a down hole tool,such as an MDT.

To detect phase changes of complex hydrocarbon mixtures, it is necessaryto nucleate bubbles or drops of the new phase and to detect thesebubbles. In standard laboratory apparatus, and in prior art down holetools, the bubbles or drops are formed at arbitrary locations in thefluid volume, and then detected by pressure-volume measurements, or bydetecting bubbles at another site (e.g., in the beam between a sourceand detector of light). Both of these methods are characterized by adelay between the arrival at the thermodynamic phase line and theinitiation of phase change, and then a delay between the phase changeand its detection.

The solution proposed by this invention is to use an ultrasonictransducer in the fluid flowline to create bubbles by cavitation.Cavitation, however, is generally considered to be impossible when fluidpressure is high. Although several hundred psi is a rule of thumb fortypical piezoelectric ultrasonic transducers, the pressure in thesampling tool flowline is as high as 20,000 psi. Therefore, it wouldappear that cavitation is not a viable method of creating bubbles downhole. However, for a fluid at the bubble point (i.e., the point at whichbubbles are thermodynamically stable, but form slowly), modest localizedpressure reductions, such as are found in acoustic waves, can lead toefficient evolution of bubbles.

The bubbles thus formed are detected at the site where they are producedby monitoring the ultrasonic properties of the liquid. This isconveniently done by monitoring the acoustic impedance of the ultrasonictransducers used to cavitate the fluid. At the first appearance of abubble, even a transient bubble, the acoustic impedance mismatch betweentransducer and fluid is greatly altered. This in turn produces a changein the electrical impedance of the transducer.

Another method of nucleating bubbles at the bubble point is to providepredetermined locations in the fluid volume at which the temperaturediffers incrementally from that of the main body of liquid. For ordinaryliquids, bubbles are preferentially formed where local hot spots occurin the liquid.

Crude oils differ from ordinary liquids in that they can have unusualphase diagrams. For some crudes, bubbles form preferentially at coldspots in the liquid volume. Thus, in order to be certain that the bubblepoint is accurately measured for all kinds of crude oils and crude oilmixtures, both a hot and a cold spot should be provided. A transducerplaced in proximity to these hot and cold locations can sensitivelydetect the first appearance of bubbles.

No strong signature appears in the P-V characteristic at the dew point,because the first appearance of liquid does not substantially change thecompressibility of the mixture. Therefore, it is necessary to sense theliquid phase directly. To do this, the first condensation must be on amoisture sensor.

Dew point sensors are normally integrated with coolers so that the firstcondensation occurs on the sensor. However, mixtures of hydrocarbonsfound in reservoirs can have unusual phase diagrams: condensation canoccur at the hottest point accessible to the mixture. Amyx, Bass,Whiting, “Petroleum Reservoir Engineering”, McGraw-Hill, 1960, pp.220-229. Thus, moisture sensors must be mounted on both a heater and acooler in order to ensure that the dew point will be measured accuratelyunder all circumstances.

The method of the invention consists of the following steps:

a) withdrawing a fluid sample from the formation fluid using a formationsampling tool, such as an MDT;

b) closing valves in a flowline of the formation sampling tool in orderto establish a well-defined sample volume;

c) expanding this sample volume in step-by-step fashion (i.e.,incrementally moving a piston of the pumpout module of the formationsampling tool);

d) nucleating bubble formation or a drop of liquid at a predeterminedsite in the sampled volume;

e) observing an onset of bubble formation or a drop of liquid at thepredetermined site; and

f) measuring pressure of fluid at the onset of bubble formation or adrop of liquid, which pressure measurement defines the bubble point orthe dew point.

BRIEF DESCRIPTION OF THE DRAWINGS

A complete understanding of the present invention may be obtained byreference to the accompanying drawings, when considered in conjunctionwith the subsequent detailed description, in which:

FIG. 1 illustrates a schematic view of a typical MDT tool that can beused for practicing the methods of the invention;

FIG. 2 depicts a graph of a phase diagram of a gas condensate reservoir;

FIG. 3 shows a graph of a phase diagram of a crude oil with significantdissolved gas;

FIG. 4 illustrates a graph of a P-V curve for a complex hydrocarbonmixture at constant temperature, wherein no distinct slope change occursat the bubble point;

FIG. 5 depicts a graph of a P-V curve for a complex hydrocarbon mixtureat constant temperature, wherein a distinct slope change occurs at thebubble point;

FIG. 6 is a schematic diagram of a simple electrical circuit formonitoring transducer impedance;

FIG. 7 illustrates a flow diagram for the method of bubble pointmeasurement in accordance with the invention;

FIG. 8 depicts a schematic diagram of an apparatus for measuring the dewpoint in accordance with the method of the invention; and

FIG. 9 shows a flow diagram for the method of dew point measurement inaccordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Generally speaking, the present invention features a method ofdetermining the bubble point and dew point of formation fluids downhole. Extracting fluids from earth formations by means of samplinglogging tools is widely known and practiced. “Schlumberger WirelineFormation Testing and Sampling” (1996). The best known commercial toolsused for this purpose are the Schlumberger Modular Formation DynamicsTester (MDT) and the Western-Atlas Reservoir Characterization Instrument(RCI).

Now referring to FIG. 1, a typical MDT tool 10 having a PVT module 11 isshown.

For purposes of definition herein, tools that extract fluids fromformations are generically called “sampling tools”. Most commonly,sampling tools pump formation fluid for a substantial period of time inorder to minimize contamination by mud filtrate. The MDT tool 10 has apumpout module 12 for this purpose. During the pumping process, fluidproperties are measured by various means, such as low-frequencyelectrical conductivity (MDT and RCI), dielectric constant (RCI) and/oroptical properties (MDT). In the initial stage, this fluid is discardedby being pumped either into the borehole or back into the formation at aremote point. The fluid is redirected to one or more sample bottles inthe sample module 14; subsequently, the fluid is transported in suchbottles to the surface for extensive examination and testing, whencontamination has been minimized. Alternatively, measurements of bubblepoint can be made inside the tool by the aforementioned patentedmethods.

There are two main problems with prior art, down hole bubble point anddew measurements: the measurements are slow, and the measurements areinaccurate. The bubble point or dew point measurements are relativelytime consuming. The bubble point measurement is impeded by bubbles thatdo not readily form at the thermodynamic bubble point of the liquid.Even when the gas phase is thermodynamically stable at a giventemperature and pressure, a gas bubble may be unable to form because itssurface free energy exceeds the free energy difference of the bulkphases. This phenomenon accounts for supercooling or superheating and isgenerally observed at first order phase transitions, described byclassical nucleation theory. A. W. Adamson, “Physical Chemistry ofSurfaces”, 3rd edition, Wiley, chap. 8, 1976. In order to minimize theerror associated with nucleation, bubble point measurements are made bychanging the volume very slowly, typically over an hour.

Chemists have found that liquid-to-gas transitions can be observed morereproducibly when the liquid is stirred, but implementing that techniquein the flowline of a down hole sampling tool would compromisereliability. Thus, the stirring procedure is not a preferred solution.

Referring to FIG. 2, there is shown a typical phase diagramcharacterizing a gas condensate reservoir. The horizontal axis istemperature and the vertical axis is pressure. When a reservoir is firstpenetrated by a borehole, the reservoir is characterized by its originaltemperature and pressure. Two possible original states are shown, atPoints 1 and 2. To bring the reservoir into production, the pressure isreduced at constant temperature. Thus, reservoir production isrepresented by movement down vertical lines in FIG. 2.

In order to maintain maximum permeability to hydrocarbon flow, it isessential that only one fluid phase exist in the formation. This meansthat the pressure must remain above the Dew Point line shown in FIG. 2.Above this line, only gas exists; below the Dew Point Line, liquidcondenses, forming a two-phase mixture in the rock pores of the earthformation. The presence of two phases decreases permeability to fluidflow, and therefore reduces production rate.

To detect the dew point pressure at down hole temperature using a fluidsampling tool, a sample of formation fluid is drawn into the tool at apressure as close to formation pressure as possible. The sample in thetool is then isolated and the pressure reduced in a controlled manner,as described herein. When the dew point is reached, liquid condenses.

Ordinary dew point sensors used to measure atmospheric humidity arethermostated at a temperature slightly below the ambient temperature.The same technique is appropriate for reservoirs characterized byinitial temperature and pressure conditions exemplified by Point 2 (FIG.2). Condensation first appears at the cooled sensor, giving a reliablemeasurement of the dew point.

However, for gas condensate reservoirs characterized by initialconditions exemplified by Point 1 (FIG. 2), prior art sensors yielderroneous results. In that case, the cooled sensor is the last place inthe volume on which liquid condenses. Therefore, it is necessary for thesensor to be placed at the warmest point in contact with the fluid to betested. Liquid first condenses on the warm sensor, which thereforedetects the first droplet of liquid resulting from the pressurereduction.

Referring now to FIG. 3, a typical phase diagram is illustrated of acrude oil reservoir with significant dissolved gas content. Once again,the horizontal axis is temperature and the vertical axis is pressure.When a reservoir is first penetrated by a borehole, the reservoir ischaracterized by its original temperature and pressure. Two possibleoriginal states are shown, at Points J and K. To bring the reservoirinto production, the pressure is reduced at a constant temperature.Thus, reservoir production is represented by movement down verticallines in FIG. 3.

As aforementioned, in order to maintain maximum permeability tohydrocarbon flow, it is essential that only one fluid phase exist in theformation. This means that the pressure must remain about the BubblePoint Curve shown in FIG. 3. Above this line, gas is completelydissolved in the oil; below the Bubble Point Curve, gas comes out ofsolution, forming a two-phase mixture in the rock pores of the earthformation. The presence of two phases decreases permeability to fluidflow, and therefore reduces production rate.

To detect the bubble point pressure at down hole temperature using afluid sampling tool, a sample of formation fluid is drawn into the toolat a pressure as close to formation pressure as possible. The sample inthe tool is then isolated and the pressure reduced in a controlledmanner as described herein. When the bubble point is reached, free gasappears in the oil.

For many fluid mixtures, bubbles first appear in the fluid at thehottest point in the volume. In these fluids, a heater can be used tonucleate gas at a predetermined location. The same technique isappropriate for reservoirs characterized by initial temperature andpressure conditions exemplified by Point J (FIG. 3).

However, for those reservoirs characterized by initial conditionsexemplified by Point K in FIG. 3, the warmest point is the last place inthe volume at which bubbles form. Therefore, it is necessary for thebubble sensor to be placed at the coldest point in contact with thefluid to be tested.

Cavitation avoids the need to provide hot or cold points in bubble pointcells. Bubbles first form at the location where sonic amplitude isgreatest. Bubbles at the same place are readily detected by sonic means.

Referring to FIG. 5, it will be observed that for a complex hydrocarbonmixture at constant temperature, a distinct slope change may occur atthe bubble point. However, this may not always be the case, as seen bythe pressure-volume curve illustrated in FIG. 4.

The solution proposed by this invention is to use an ultrasonictransducer to create bubbles by cavitation. Cavitation, however, isgenerally considered to be impossible when fluid pressure is high.Although several hundred psi is a rule of thumb for typicalpiezoelectric ultrasonic transducers, the pressure in the sampling toolflowline is as high as 20,000 psi. Therefore, it would appear thatcavitation is not a viable method of creating bubbles down hole.However, for a fluid at the bubble point (i.e., the point at whichbubbles are thermodynamically stable, but form slowly), modest localizedpressure reductions, such as are found in acoustic waves, can lead toefficient evolution of bubbles.

Various means may be used to induce cavitation, such as flowrestrictions and propellers. The ultrasonic method is particularlysuitable for sampling tools. The transducer may form part of the wall ofthe flowline. Deployed in such a manner, it does not interfere withother objectives of the sampling tool that rely on the unimpeded flow offluid through the flowline. It is also relatively immune from erosionand has no moving parts, which are important considerations in down holetools.

It is as important to sense the presence of bubbles as it is to generatethem. Laboratory studies have shown that the pressure versus volumecurve can be an unreliable bubble point indicator for many crude oils,as aforementioned. Thus, means (e.g., optical means) have been devisedto sense the presence of bubbles directly. Such sensors can probe only apart (often only a small part) of the total volume of fluid, so thesemeans depend on the bubbles being transported to the site of the sensor.This is one purpose of the stirring process often used in laboratories.A stirring mechanism can be a failure-prone component in a fluidsampling tool, and hence it is not included in the preferred mode oftransporting samples to the site of a bubble sensor.

The solution proposed by this invention is to sense bubbles at the siteat which they are produced. That is, bubbles are sensed at the locationof the ultrasonic transducer used for cavitation. The acoustic impedancesensed by the ultrasonic transducer is extremely sensitive to thepresence of bubbles, so bubbles can be produced and sensed at the samesite, with very high reliability. The pressure of the fluid at whichbubbles are first generated by the ultrasonic transducer is measured bya precision gauge, such as the Schlumberger CQG quartz pressure gauge.

The acoustic impedance of a material is defined as the product of itsmass density and sound speed. In one implementation of the invention,the acoustic impedance of the transducer is approximately matched to theacoustic impedance of the fluid, in the absence of bubbles. At the firstappearance of a bubble, both the density and the sound speed of thefluid decrease. The transducer and fluid are no longer impedance matchedacoustically. Under this condition, the electrical impedance of thetransducer increases.

Referring to FIG. 6, there is shown a simple electrical circuit used tomonitor the electrical impedance of the transducer. An electronicoscillator 101 drives alternating current through a resistor 102 (havingfixed resistance, R) and an acoustic transducer 103. Transducer 103radiates sound energy into fluid 104.

The current in the circuit, I, is monitored by using a high-impedancevoltmeter 105 to measure the voltage, V_(r), across resistor 102. Ohm'sLaw states that I=V_(r)/R.

The voltage across transducer 103, V_(t), is monitored by a secondvoltmeter 106. The electrical impedance of the transducer 103 isZ=V_(t)/I=(V_(t)/V_(r))R.

When the acoustic impedance of the transducer is matched to the acousticimpedance of the fluid, in the absence of bubbles, the voltage acrossthe transducer is relatively low; the current is relatively high. Thus,the electrical impedance of the transducer is relatively low.

When the acoustic impedances of transducer and fluid are mismatched,however, in the presence of bubbles, the voltage across the transducerincreases and the current decreases, increasing the electricalimpedance.

Referring to FIG. 7, a flow chart 20 is illustrated for the method ofmaking a bubble point measurement, in accordance with the invention. Thedown hole fluid that is free of contamination is admitted into the tool,step 22. A valve in the tool is closed, step 24, in order to define agiven volume. An ultrasonic transducer or other cavitation means is thenenabled, step 26. The pressure and temperature of the sample fluid ismeasured, step 28. Then the transducer is monitored to detect thepresence of a bubble, step 30. If the bubble is detected, the bubblepressure is recorded, step 40. The cavitation source is then disabled,step 42, and the sampled fluid is expelled to the borehole, step 44. Ifa bubble is not detected for the given pressure and temperature, step30, then the volume is increased by moving the piston of the samplingmodule, step 36. The sample is then remeasured for pressure andtemperature, step 28. The detection process, defined by steps 30 through44, is then repeated.

Referring to FIG. 8, an apparatus 50 for measuring the dew point downhole is illustrated. The fluid being sampled is drawn into a chamber 52through a flow line 54 and inlet valve 56. The pressure gauge 58measures the pressure in chamber 52. The pressure in the chamber 52 canbe adjusted by piston 51. The temperature is also measured by suitablemeans (not shown). The Peltier cooler 60 reduces the temperature of thefluid at a selected site in chamber 52, while the heater 62 raises thetemperature at another site. Liquid sensors 64 disposed at each site areused to detect the formation of a drop of liquid. After the measurementsare taken, the sample is discharged to the borehole through the outletvalve 66 and flowline 68.

Referring to FIG. 9, the method of measuring the dew point in accordancewith the invention is illustrated by the flow chart 80. The fluid isadmitted into the chamber 52, step 82. The valve 56 is closed to definethe volume in chamber 52, step 84. The heater 62 and the cooler 60 areenabled, step 86. Pressure and temperature are measured, step 88. Thesensors 64 monitor the presence of a liquid drop, step 90. If dropletsare detected, then the dew point is recorded, step 94, the heater andcooler are disabled, step 96, and the fluid sample is expelled to theborehole, step 98. If no droplets are detected, step 90, then the volumeof the fluid in chamber 52 is increased, step 93, and pressure andtemperature are again measured, step 88. Then steps 90 through 98 arerepeated.

Since other modifications and changes varied to fit particular operatingrequirements and environments will be apparent to those skilled in theart, the invention is not considered limited to the example chosen forpurposes of disclosure, and covers all changes and modifications whichdo not constitute departures from the true spirit and scope of thisinvention.

Having thus described the invention, what is desired to be protected byLetters Patent is presented in the subsequently appended claims.

What is claimed is:
 1. An in situ method of fluid analysis in the borehole of a well for determining phase characteristics of a formation fluid, comprising the steps of: a) withdrawing a fluid sample from said formation fluid using a formation sampling tool; b) establishing a well-defined sample volume; c) incrementally expanding the sample volume of step (b); d) nucleating bubble formation in said sample volume at fluid pressures greater than approximately 400 psi and approaching at least up to 20,000 psi by generating cavitation at a predetermined site; e) detecting an onset of bubble formation at said predetermined site; and f) measuring pressure of fluid at the onset of bubble formation in accordance with step (e), which pressure measurement defines a phase change of said formation fluid.
 2. The method of fluid analysis of a formation fluid in accordance with claim 1, wherein said incrementally expanding the sample volume in accordance with step (c) further comprises the step of: g) incrementally moving a piston of a pumpout module of the formation sampling tool.
 3. The method of fluid analysis of a formation fluid in accordance with claim 1, wherein said detection of detection step (e) is provided by an ultrasonic transducer.
 4. An in situ method of fluid analysis in the borehole of a well for determining phase characteristics of a formation fluid, comprising the steps of: a) withdrawing a fluid sample from said formation fluid using a formation sampling tool; b) establishing a well-defined sample volume; c) incrementally expanding the sample volume of step (b); d) nucleating liquid drop formation in said sample volume at fluid pressures greater than approximately 400 psi and approaching at least up to 20,000 psi by generating cavitation at a predetermined site; e) detecting an onset of liquid drop formation thereby defining a dew point condition at said predetermined site; and f) measuring pressure of fluid at the onset of liquid drop formation in accordance with step (e), which pressure measurement defines a phase change.
 5. The method of fluid analysis of a formation fluid in accordance with claim 4, wherein said incrementally expanding the sample volume in accordance with step (c) further comprises the step of: g) incrementally moving a piston of a pumpout module of the formation sampling tool.
 6. An apparatus for determining a phase change in a fluid sample down hole of a borehole characterized by high temperature and high pressure conditions, comprising; flow sampling means for taking extracting a fluid sample down hole; means associated with said sampling means for creating a phase change in said sample at fluid pressures greater than approximately 400 psi and approaching at least up to 20,000 psi at a given location; and detection means for detecting said phase change at at said given location.
 7. The apparatus in accordance with claim 6, wherein said detecting means comprises an ultrasonic transducer.
 8. The apparatus in accordance with claim 7, wherein said ultrasonic transducer comprises a piezoelectric ultrasonic transducer.
 9. The apparatus in accordance with claim 6, wherein said means for creating a phase change comprises an ultrasonic transducer.
 10. The apparatus in accordance with claim 9, wherein said ultrasonic transducer comprises a piezoelectric ultrasonic transducer.
 11. An apparatus for determining a phase change in a fluid sample down hole of a borehole characterized by high temperature and high pressure conditions, comprising: sampling means for taking extracting a fluid sample down hole; means associated with said sampling means for creating a phase change in said sample at fluid pressures greater than approximately 400 psi and approaching at least up to 20,000 psi by generating bubbles at a given bubble generating location in said apparatus; and detection means for detecting said phase change of said sample at said given bubble generating location.
 12. The apparatus in accordance with claim 11, wherein said detecting means comprises an ultrasonic transducer.
 13. The apparatus in accordance with claim 12, wherein said ultrasonic transducer comprises a piezoelectric ultrasonic transducer.
 14. The apparatus in accordance with claim 11, wherein said means for creating a phase change comprises an ultrasonic transducer.
 15. The apparatus in accordance with claim 14, wherein said ultrasonic transducer comprises a piezoelectric ultrasonic transducer.
 16. The method of fluid analysis in accordance with claim 4, wherein the predetermined site is heated.
 17. The method of fluid analysis in accordance with claim 4, wherein the predetermined site is cooled.
 18. The apparatus in accordance with claim 6, wherein the means for creating a phase change comprises a heater.
 19. The apparatus in accordance with claim 6, wherein the means for creating a phase change comprises a cooler.
 20. The apparatus in accordance with claim 6, wherein the means for creating a phase change comprises means for adjusting the pressure of the sample. 